The present invention relates to a plasma jet generator for use in DC (direct current) plasma spectrochemical analysis, and more particularly to a plasma jet generator wherein electrodes can be replaced by using a manipulator, which is useful for, e.g., analysis within a hot cell or a glove box in research in the nuclear power field, although it is not especially limited thereto.
Analyses in research on high-level radioactive materials in fields related to nuclear power must be remote-controlled analyses employing a hot cell shut off from their surroundings. However, since conventional analyses such as titration, colorimetric and similar analyses are analytical operations originally intended to be carried out in laboratories, it is difficult to apply these analyses to in-cell analytical methods. In addition, these analyses have a disadvantageously complicated analytical operation and use a wide variety of reagents although the range of subjects to be analyzed is narrow. Moreover, a long period of time is required for these analyses.
Emission spectrochemical analysis wherein an analytical sample is excited so as to emit light, and the emission spectrum wavelengths and the spectral line intensities are measured to identify the components of the analytical sample and their concentrations is simple in operation, has a high detection sensitivity, and can measure a wide range of concentrations, so that it provides the advantage that several elements can be analyzed simultaneously. Moreover, since the principle of emission spectrochemical analysis is based on the measurement of light, it is possible to effect stable measurements independent of high-level radioactive rays which are a large obstructive factor in hot cells. Therefore emission spectrochemical analysis is the most suitable as an in-cell analytical method.
A typical DC plasma emission spectrometer, as shown in FIG. 1, comprises a DC plasma jet generator (light-emitting unit) 1, a spectroscope 2, a detection unit 3 and a data processing unit, as well as other elements. This DC plasma emission spectrometer is a conventionally-known simultaneous multi-element instrument wherein an analytical sample is introduced into a high-temperature DC plasma so that it is excited to emit light, and its emission spectral lines are detected by a spectroscope in order to identify and quantify the elements in the sample from the spectral lines and their intensities. Incidently, reference numeral 4 in FIG. 1 designates an outlet slit placed in the focal plane of the spectrum.
In this case, the DC plasma jet generator 1 has a construction, as clearly shown in FIG. 2, such that two anode blocks 10 and one cathode block 11 are positioned in an inverted Y shape. A graphite electrode 12 (e.g., of dimensions 2.3 mm diameter and 46.6 mm length) is employed as each of the anodes, while a tungsten electrode 13 (e.g., of dimensions 1 mm diameter and 61 mm length) is employed as the cathode. Reference numeral 14 denotes ceramic sleeves mounted over each electrode block. To ignite the plasma, argon is blown in from the electrode blocks 10 and 11, and the graphite electrodes 12 and the tungsten electrode 13 are pushed out from their respective ceramic sleeves 14 so as to come in contact with each other and make a discharge. When the plasma has ignited, the three electrodes are withdrawn into their respective ceramic sleeves 14, and the ionization of the argon is maintained to form a plasma 15. An analytical sample nebulized by a nebulizer (not shown) is introduced into the plasma 15 through a sample-introducing tube 16 made of quartz, and is thus excited to emit light.
A typical conventional anode block is shown in FIG. 3 in detail. The anode block comprises a cylinder 20, a water-cooled cooling block 21 mounted on the end of the cylinder 20, a piston 22 reciprocatable in the cylinder 20, a spring 23 housed in the cylinder 20 together with the piston 22, and the ceramic sleeve 14 attached to the end of the cooling block 21. The piston 22 is advanced by the pressure of the argon blown in from the base end or the proximal end of the cylinder 20, and is retracted by the force of the coiled spring 23 when the supply of argon is stopped. A recess 24 formed in the distal end of the piston holds the graphite electrode 12. The graphite electrode 12 is inserted into the piston end recess 24 and is rotated through about 90 degrees, so that a cut portion 25 at the proximal end of the graphite electrode 12 is secured therein. The removal of the graphite electrode 12 is effected by a similar manual operation, i.e., by rotating it through 90 degrees. These operations depend a great deal on the sensitivity of the operator. The ceramic sleeve 14 is removed whenever the electrode is replaced.
This conventional DC plasma emission spectrometer is an analytical instrument which is effective when used in an ordinary laboratory, as mentioned before. It is, however, impossible to carry out any analysis by installing such an instrument in a hot cell without modification. This is because the interior of a hot cell is a bad environment which has both a high temperature and a high humidity and includes acid vapors, etc., which presents problems in the maintenance and management of the instrument. Another reason is that it is difficult to effect the operation and maintenance of the elements and members of the instrument using manipulators. Accordingly methods have been examined in which the spectroscope, data processing unit, etc., including the complicated optical systems and electrical circuits, are installed outside the cell, the DC plasma jet generator is installed inside the cell, and the optical data on the inside of the cell is extracted out of the cell. In such a case, the electrodes must be replaced periodically since they wear out during the operation of the plasma jet generator. Since it is impossible to check the degree of wear of each electrode when a DC plasma jet generator is operating in a hot cell, the electrodes must be replaced virtually every day (every time the instrument is used). However, the electrode parts are very slender, and in particular the replacement of the graphite electrodes involves an extremely high probability of breaking them since the fragile graphite is removed and attached mechanically. Therefore the replacement operation requires some training, as it does even in an ordinarly laboratory. Moreover, it is usually impossible to remove a broken electrode without disassembling the associated anode block. Although, in order to operate a plasma jet generator in a hot cell, it is necessary to make it possible to replace the electrodes required for the generation of the plasma by remote control using manipulators, such remote control is impossible in the prior art described above.